•Could
work it out “classically” by strength of forces
–Classical
mechanics doesn’t work at this scale – Need quantum mechanics
–Strength
of nuclear forces not originally known
–
•Use
E=mc2 to do
accounting
•
–Mass
is a measure of the energy stored in a system
–Loss
of mass from a system means release of energy from that system
–
•Compare
mass of four 1H to mass of one 4He
–6.693
´ 10-27 kg - 6.645 ´ 10-27 kg = 0.048 ´ 10-27 kg drop in
mass
–E
= mc2 = 0.048 ´ 10-27 kg ´ (3 ´ 108 m/s)2 = 0.43 ´ 10-11 kg m2/s2 = 0.43 ´ 10-11 J
(note == a Joule is just
shorthand for kg m2/s2)
–So
4.3 ´ 10-12 J of energy released
•This
is huge compared to chemical energy:
2.2 ´10-18 J to ionize hydrogen